This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application METHOD FOR ADJUSTING PRINTING DENSITY IN PRINTER filed with the Korean Industrial Property Office on Jun. 30, 1999 and there duly assigned Ser. No. 25720/1999.
1. Field of the Invention
This invention relates to a method for adjusting print density in a printer. More specifically, the present invention is capable of enhancing print quality of a print control language 6 (PCL6) printer or a PostScript mode printer which can perform halftone process therein by applying different threshold tables to a halftone process according to changes in peripheral environment (e.g., temperature and humidity) of an engine of the printer.
2. Description of the Related Art
Generally, as one of the image forming devices using the electrophotographic developing method, a printer includes a pickup roller, a charging roller, an organic photoconductive (OPC) drum, a transferring roller, a developing roller, a supplying roller, a fixing unit and an exposing device in order to print a predetermined image data on a paper.
The charging roller, which is charged with a high voltage, is rotated and evenly charges a photosensitive body on the outer periphery of the OPC drum. At this time, light emitted from the exposing device forms an electrostatic latent image on the outer periphery of the charged OPC drum. Then, a potential difference occurs between the supplying roller supplied with a high potential supplying voltage and the developing roller supplied with a lower level voltage than the supplying roller. Accordingly, negative charges move from the supplying roller 106 to the developing roller. In this manner, toner is supplied from the developing roller and coated on the electrostatic latent image formed on the outer periphery of the OPC drum so that the electrostatic latent image is changed into a visible image. The transferring roller at high potential transfers the visible image formed on the OPC drum to a transferred paper. The transferred visible image is fixed on the paper by the heat and pressure of a heating roller and a pressing roller of the fixing unit. This completes this printing process by the printer.
The supplying voltage, the developing voltage, the transferring voltage, and the charging voltage described above are continuously supplied to the supplying roller, the developing roller. the transferring roller and the charging roller, respectively. And the heating roller in the fixing unit is maintained in the turned-on state until the printing process is terminated.
Moreover, a pre-transfer lamp (PTL) is located between the developing roller and the transferring roller. The PTL generates light having a wavelength of 650 nm and applies the light to the OPC drum. Then, the electric potential of the toner coated on the OPC drum is lowered and the bonding force of the toner to the OPC drum is reduced. As a result, transfer efficiency is enhanced.
On the other hand. the method for adjusting the lightness of an image is largely divided into two methods: one is a method for adjusting the lightness of the image in a printer drive program; and the other is a method for directly adjusting the lightness of the image in a printer engine.
In the case that the printer is a type having a translatable language mode below PCL 5 mode, the user can adjust the image density in the printer drive program of the computer. Therefore, the user can adjust print density through an ECONO mode (for economizing on toner), a RET mode and so on in the printer drive program. In the xe2x80x9cecono modexe2x80x9d, the amount of toner corresponding to each pixel is entirely reduced, thereby saving the toner. In the xe2x80x9cret modexe2x80x9d, the toner is not provided in some of pixels making up a spot, thereby saving the toner. Moreover, since the halftone process is performed in the printer drive program of the computer in the case of a printer below the PCL 5 mode, changes in the peripheral environment cannot be actively coped with and all the halftone processes in the printer are performed in the same manner. As a result, the print quality varies with the changes in the peripheral environment.
There is a general analog controlling method through a printer engine part. According to this method, the lightness of the image is adjusted by adjusting a variable resistance value controlled by a knob, which varies the charging voltage, the developing voltage, and the transferring voltage and so on. This method, however, suffers from the inconvenience that the knob attached to the printer needs to be directly adjusted by the user. Furthermore, since the adjustment is made by determination of the user. it is difficult to obtain uniform print quality.
On the other hand, a printer having PCL6 as a translatable language or using a PostScript mode can perform the halftone process therein. Accordingly, the process load of the computer is reduced. At this time, the central processing unit (CPU) of the printer includes a threshold table stored therein. The threshold value in the threshold table and a lightness value of the image information to be presently processed are compared for each pixel. When the lightness value of the image information is larger than the threshold value, the lightness value is assigned as 0 and when the lightness value of the image information is smaller than the threshold value, the lightness value is assigned as 1. Then, the halftone process is performed according to the determined values. Therefore, the lightness of the image is determined according to the respective threshold values of the threshold table.
However, several problems occur in such a conventional halftone process of the printer. The print quality of the printer is very sensitive to changes in the peripheral environment (e.g., temperature and humidity). Particularly, since the resistance between the paper and the transferring roller is increased in a low temperature and low humidity environment, the image is printed more lightly and the image density is lower than when printed in a normal environment.
The resistance between the paper and the transferring roller is correspondingly reduced in a high temperature and high humidity environment. As a result. the image is printed more darkly and the image density is heightened compared to that printed in a normal environment. However, since the threshold value for the halftone processing is fixed in the threshold table, it is impossible to compensate for such changes in the peripheral environment and accordingly the print density (i.e., lightness of the image) varies with the environmental changes. This results in deteriorated print quality.
It is therefore an object of the present invention to provide an improved halftone printing method.
A further object of the invention is to provide a halftone printing method which compensates for print density changes due to environmental changes.
A yet further object of the invention is to provide a halftone printing method which does not require high memory capacity.
These and other objectives are met in the present invention, which provides a method for adjusting print density and increasing print quality of a printer by fixing a plurality of threshold tables corresponding to the environmental changes, applying a different threshold table according to detected environmental changes such as temperature, humidity and so on, and uniformly maintaining the image density even though the environment changes.
The above and other objects are provided according to the present invention by providing a method for adjusting a print density of a printer which includes at least a threshold table having different threshold values corresponding to environmental states of a printer engine stored therein and is capable of performing a halftone process therein, the method including the steps of. when a print operation command is received, receiving and storing print data to be printed; detecting present environmental state information in the printer engine and receiving the present environmental state information from the printer engine; selecting a threshold table corresponding to the present environmental state information among the threshold tables; determining a lightness of the print data through the selected threshold table and performing the halftone process: and transmitting the print data that is image-processed through the halftone process to the printer engine to be printed.
Preferably, the environmental state includes at least present temperature and present humidity.
Moreover, it is preferable that the environmental state is divided into three states of a high temperature and high humidity state, a low temperature and low humidity state and a normal state.